Reverse-Brayton cryocoolers use a recuperative heat exchanger to cool the high pressure gas with the cold, low pressure gas returning from the cold end. In typical reverse-Brayton cryocoolers having plate-fin design, the energy transfer in the heat exchanger is an order of magnitude or more greater than the overall cryocooler input power. Therefore, losses in the heat exchanger have a strong influence on the total input power required.
Input power reduction can be achieved by increasing the thermal effectiveness (ratio of temperature difference between the incoming and outgoing first fluid streams to temperature difference between incoming first and second fluid streams) of the heat exchanger. Unfortunately, with known plate-fin heat exchangers, it is difficult to achieve effectiveness levels in excess of about 96-97%. By increasing effectiveness to 99% or more and reducing the pressure drop ratio (pressure loss divided by system pressure) to 0.02, the input power to the cryocooler could likely be reduced by a factor of 2. In plate-type heat exchangers, it is known to form multiple concavo-convex structures, i.e., "dimples," in the sheets of material used to manufacture fluid channels in the heat exchanger. See, for example, the heat exchangers in U.S. Pat. Nos. 2,281,754 to Dalzell and 2,596,008 to Collins. These dimples provide mechanical integrity to the fluid channels. In addition, these dimples are provided for the purpose of inducing turbulent flow in the fluid channels so as to enhance convective heat transfer.
Plate-type heat exchangers have fluid channels arranged so that different fluids in adjacent channels flow in the same direction (i.e., have parallel flow fluid paths), flow in opposite directions (i.e., have counterflow fluid paths), flow in transverse directions (i.e., transverse flow fluid paths) or have a combination of these fluid flow paths. In yet another class of plate-type heat exchangers, different fluids are transported in a circumferential flow about a central axis. U.S. Pat. No. 840,667 to Speed et al. describes a circumferential, counterflow heat exchanger, and U.S. Pat. No. 5,078,209 describes a circumferential flow heat exchanger featuring both parallel flow and counterflow fluid paths.
Known recuperative plate-type heat exchangers typically include structures such as fins and plates made from a material, e.g., aluminum, having a relatively high thermal conductivity. Such structures are often configured and positioned so as to provide a relatively low resistance thermal conductivity path between inlet and outlet for a given fluid circuit. In view of these attributes of known plate-type heat exchangers, heat exchange effectiveness is typically not as high as desired.
For a given heat exchanger application, a number of design parameters, such as fluid path height and length, need to be addressed in designing an appropriate heat exchanger. When fluid properties change significantly during travel through the heat exchanger, it may be necessary to change one or more of these design parameters at various regions of the heat exchanger to maintain optimal performance. Together, these factors virtually necessitate original design of a heat exchanger for a given application, particularly when simultaneous high heat transfer effectiveness and low pressure losses are desired. Such original design adds to the time and cost associated with implementing a heat exchanger in a given application.